Science oft resembles the federal tax code: the rules are rigid, but they also keep changing. So it has been with the study of neurogenesis, or the creation of neurons in the human brain. Not long ago a hard-and-fast rule held that neurons could neither divide nor emerge from elsewhere. The neurons you were born with, in short, were the ones you took to your grave. That dogma began to change in the 1980s, however, when Fernando Nottebohm of the Rockefeller University discovered that neurons were dividing in the forebrains of canaries. It appeared that neurons divided after all.